The present disclosure relates to a gas sensor and, more particularly, to a highly sensitive gas sensor with a gas-sensitive layer of metal oxide that works at normal temperature.
Among known gas sensors are those which have a gas-sensitive layer which changes in its physical properties upon contact with a detectable gas, thereby achieving gas detection. The one having a gas sensitive layer of metal oxide semiconductor finds general use as gas leak alarms.
Any gas sensor with tin oxide (SnO2) to detect a reducing gas, such as combustible gas, works on the principle illustrated in FIGS. 20A and 20B. The gas sensor 100 consists of the alumina substrate 101, the counter electrodes 103 formed thereon, and the gas sensitive layer 102 of tin oxide semiconductor which covers the counter electrodes and fills the space between the counter electrodes. The gas-sensitive layer 102 is connected in series to the current detecting resistance 104 and the circuit power source 105 through the counter electrodes 103. The current detecting resistance 104 generates the output voltage 107 across its terminals to indicate the circuit current 106 flowing through the gas sensitive layer 102.
The gas-sensitive layer 102 adsorbs the oxygen molecules 109 on its surface because the tin oxide (SnO2) as its constituent is an n-type semiconductor compatible with oxygen in air. The adsorbed oxygen molecules 109 constrain the conduction electrons 108b near the surface, thereby forming the surface depletion layer 102b near the surface. This results in a hindrance to the conduction of electrons in the gas-sensitive layer 102. Incidentally, FIGS. 20A and 20B have been drawn for brevity on the assumption that the gas-sensitive layer 102 is a mono-crystal layer and its top surface is in contact with air.
When exposed to clean air, the gas-sensitive layer 102 adsorbs at a maximum the oxygen molecules 109 on its surface, as shown in FIG. 20A. As the result, the surface depletion layer 102b reaches its maximum thickness, with the gas-sensitive layer 102 increasing in resistance and the circuit current 106 decreasing.
By contrast, when exposed to air containing a reducing gas (such as alcohol and hydrogen), the gas-sensitive layer 102 has the adsorbed oxygen molecules 109 thereon partly removed by reaction with the reducing gas molecules 111, as shown in FIG. 20B. The removed oxygen molecules 110 release the bound conduction electrons 108b, allowing them to function as the conduction electrons 108a. As the result, the surface depletion layer 102b decreases in thickness and the gas-sensitive layer 102 decreases in resistance, thereby causing the circuit current 106 to increase. Thus, the increase in electron conduction (or the decrease in resistance) indicates the concentration of the reducing gas molecules 111.
For enhancement of detection sensitivity and gas selectivity, the gas sensor 100 is usually incorporated with a noble metal catalyst (not shown) in or on the gas-sensitive layer 102. This catalyst promotes the surface reaction between the adsorbed oxygen 109 and the reducing gas 111. The gas sensor 100 with a low sensitivity at normal temperature is also provided with the heater 116 to heat it to about 300° C. to promote the surface reaction.
The gas-sensitive layer 102 greatly changes in conductivity depending on the concentration of the reducing gas molecules 111 as mentioned above. However, this change takes place only in the region where the surface depletion layer 102b forms. Thus, conductivity changes only slightly in the inner bulk layer 102a. For the change in concentration of the reducing gas molecules to be detected in terms of large change in the output voltage 107, it is important that the gas-sensitive layer 102 should have the bulk layer 102a in as small of a ratio as possible or should be as thin as possible.
Although it is assumed for brevity that the gas-sensitive layer 102 shown in FIGS. 20A and 20B is a mono-crystal layer, the ordinary gas-sensitive layer 113 schematically shown in FIG. 21 is a polycrystal layer composed of a large number of microcrystals 112 joined together. In this case, the following two points should be taken into account.
The first point is that the adjoining two microcrystals 112 in the polycrystal layer are in contact with each other, with the grain boundary 112c interposed between them, and hence the conduction electrons 108a flowing in the gas-sensitive layer 113 move through the grain boundary 112c. Contact at the grain boundary 112c between the two microcrystals 112 is usually made through the surface depletion layer 112b. Consequently, the conduction electrons 108a vary in the boundary potential they receive at the grain boundary 112c in response to the change in concentration of the detectable gas 114. This leads to the change in dynamic state of the conduction electrons 108a passing through the grain boundary 112c. In other words, the polycrystalline gas-sensitive layer 113 performs its gas sensing function by means of the grain boundary potential which plays an essential part. Therefore, any high sensitive gas sensor should be constructed such that the grain boundary 112c greatly changes in conductivity in response to the change in concentration of the detectable gas 114.
The second point is that the gas-sensitive layer 113 of laminate structure as shown in FIG. 21, in which the microcrystal grains 112 form multiple layers instead of a mono-layer, requires the detectable gas 114 to diffuse to its lower layer so that it performs its function.
Attempts are being made to reduce as much as possible the particle size of the microcrystal grains of semiconductor (such as tin oxide), thereby reducing the ratio of the bulk layer 102a in a thick film sensor or thin film sensor to increase its sensitivity. However, the attempts have been unsuccessful so far because the microcrystal grains with a reduced particle size in laminate structure as shown in FIG. 21 have intergrain gaps that are too small to permit the detectable gas 114 to reach the lower layer by diffusion. Moreover, the gas sensor of laminate structure has a low response speed because it prevents the detectable gas 114 from rapidly entering and leaving the gas-sensitive layer 113 by diffusion.
There has been proposed a gas sensor in Japanese Patent Publication No. Sho 62-28420 (p. 2, FIG. 2) from the forgoing point of view. This gas sensor has a gas-sensitive layer of an oxide of perovskite structure, which is formed by plasma spraying and subsequent cooling to make a large number of fine cracks. The gas-sensitive layer with fine cracks allows for easy diffusion of the detectable gas.
There has also been proposed a gas-sensitive layer of different type than mentioned above. It is not a solid in simple shape with a size of the order of micrometers but is in the form of fine filament or tube with a diameter of the order of nanometers or thin film with a thickness of the order of nanometers, so that it has a large surface area to volume ratio.
An example of such a gas-sensitive layer is disclosed in Japanese Patent Laid-open No. 2005-144569 (pp. 3 to 5, FIGS. 1 to 5 and 9). It consists of a lower layer of two-dimensionally arranged fine particles of silicon oxide (SiO2) and an upper layer of tin oxide (SnO2).
The gas-sensitive layer disclosed in Japanese Patent Laid-open No. 2005-144569 is shown in FIG. 22 (partly enlarged top view and sectional view). As shown in FIG. 22, the gas sensor 120 consists of the flat substrate 121, the silicon oxide fine particles 122 which are two-dimensionally arranged in large number, and the gas-sensitive layer 124 of tin oxide (SnO2) formed by vacuum deposition (which covers the layer of silicon oxide fine particles).
The layer of silicon oxide fine particles is formed by densely arranging the fine particles (with a diameter of about 100 nm) such that they come into contact with one another and then performing dry etching on them so that they decrease in diameter and the fine particles 122 are separated by the gaps 123 (several nanometers to tens of nanometers). The gas-sensitive layer 124 is formed on the fine particles 122, so that the fine particles 122 are covered individually by the semispherical coatings 125. The semispherical coatings 125 are joined by the bridge 126 at the gap 123 between the fine particles. The coatings 125 function in the same way as the fine crystal particles 112 of tin oxide shown in FIG. 21 and the bridge 126 functions in the same way as the grain boundary 112c shown in FIG. 21.
Japanese Patent Laid-open No. 2005-144569 mentions that the foregoing structure improves the sensitivity of the gas sensor because the gas-sensitive layer 124 is formed such that the coatings 125 in large number are joined together by the extremely fine bridges 126 and the shape of the bridge 126 is controlled by the size of the gap 123.
There is also disclosed a gas sensor of another type in Japanese Patent Laid-open No. 2002-323467 (pp. 3, 5, and 6, FIG. 3), in which the gas-sensitive layer is a polycrystalline mono-layer composed of two-dimensionally arranged microcrystal grains of metal oxide semiconductor.
The gas-sensitive layer (and the vicinity thereof) disclosed in Japanese Patent Laid-open No. 2002-323467 is shown in section in FIG. 23. According to the disclosure, the gas-sensitive layer 133, which is formed on the flat substrate 131, is a polycrystalline mono-layer of two-dimensionally arranged microcrystal particles 132 of metal oxide such as tin oxide (SnO2). The microcrystal particles 132 should have as large an average particle diameter as possible in the plane direction (which is at least larger than the average particle diameter in the thickness direction). Also, the gas-sensitive layer 133 should have a thickness of 3 nm to 12 nm, which is smaller than the thickness of the surface depletion layer which occurs when the gas-sensitive layer 133 adsorbs the detectable gas.
The gas-sensitive layer 133 is formed in the following manner. First, the substrate 131 is finished flat by mechanical polishing and then cleaned by acid or alkali washing, so that its surface irregularities are smaller than one-fifth the thickness of the gas-sensitive layer 133.
Then, the substrate 131 is coated with the gas-sensitive layer 133 by the atomic layer growing method, which consists of alternately repeated steps of supplying the substrate surface with a gas containing metal elements constituting the gas-sensitive layer 133 and supplying the substrate surface with water, every consecutive two steps forming one atomic layer on the substrate 131.
The gas-sensitive layer 133 formed on the smooth surface of the substrate 131 as mentioned above have a uniform composition and hence consists of coarse microcrystals. This is because the smooth surface and the uniform composition help form coarse microcrystals.
The thus obtained gas-sensitive layer 133 is a polycrystalline mono-layer in which there is only one particle in the thickness direction and there are limited grain boundaries 132c between the microcrystal grains 132. Japanese Patent Laid-open No. 2002-323467 mentions that the polycrystalline mono-layer and the limited grain boundaries 133 minimize the diffusion of the detectable gas into the gas-sensitive layer 133, thereby contributing to a rapidly responsive gas sensor.
The gas-sensitive layer disclosed in Japanese Patent Publication No. Sho 62/28420 is an extremely thin solid of simple shape (of the order of micrometers) having a low ratio of surface area. It makes it difficult for the detectable gas to diffuse deep into it even though it has gas diffusion paths formed therein by cracking or the like because such gas diffusion paths are very long. In addition, it lacks uniform sensing characteristics because it involves more difficulties in evenly distributing the metal catalyst within the oxide fine particles or on the particle surface as the particle size decreases.
Also, the gas-sensitive layer 124 shown in Japanese Patent Laid-open No. 2005-144569 has a high ratio of surface area but is so brittle that the silicon oxide fine particles 122 are liable to peel off from the substrate 121. Moreover, it needs a complex crosslinking process for the bridges 126 to be formed uniformly at all times regardless of the gap 123 varying in size. In addition, the fact that the gas-sensitive layer 124 does not have a flat surface is unfavorable for the metal catalyst to be distributed to desired positions.
The gas-sensitive layer 133 disclosed in Japanese patent Laid-open No. 2002-323467 is based on the idea of reducing the grain boundaries 132c as far as possible. This idea seems contradictory to the sufficient sensitivity because the grain boundaries 132c become less sensitive as they decrease. In addition, the gas-sensitive layer 133 should have an accurately controlled thickness if the resulting gas sensors are to have uniform characteristics. This objective will be achieved only at the sacrifice of many processes including highly accurate processes, low productivity and yields, and high production cost.
Finally, the gas sensors disclosed in Japanese Patent Publication No. Sho 62/28420, Japanese Patent Laid-open No. 2005-144569 and Japanese patent Laid-open No. 2002-323467 are poor in sensitivity and stability at normal temperature and hence needs heating to a high temperature. This consumes electric power and endangers their use under certain circumstances.
The present invention was completed to address the above-mentioned problems. It is desirable to provide a gas sensor and a method for production thereof, the gas sensor being highly sensitive, small in size, and capable of stable operation with low power consumption at normal temperature, and the method allowing for efficient, uniform production at low cost.